1. Field of the Invention
This invention relates to thermoelectric devices and in particular to very small thermoelectric generator devices.
2. Description of the Related Art
The generation of electricity by the application of heat at the junction of two dissimilar materials has been known for 178 years. Thermoelectric modules fabricated from simiconductor materials can convert heat into electricity with efficiencies of up to six percent. Despite efficiencies much lower than some other electric generators, these devices can have many practical applications. Waste heat is generally free; so low efficiencies may not be important. In many remote locations thermoelectric devices may be the most economical means of generating electricity even if a source of heat must be provided. Thermoelectric devices can be made very reliable and very small. Many space vehicles have their instruments powered by thermoelectric devices, which typically use heat produced by radioactive sources such as Pu-238 sources. Small prototype thermoelectric devices using radioactive sources have been produced for the purpose of being implanted in people for providing power for pacemakers. One such example is described in U.S. Pat. Nos. 3,780,425 and 3,781,176. This device was proposed as a thermoelectric battery for a cardiac pacemaker. These patents proposed a method of manufacture that proposed the use of epoxy impregnated paper insulators between thermoelectric elements and the use of a photo-resist method of applying contacts.
The United States acting through NASA and JPL are seeking to develop a small power source for a Mars vehicle. The plan is to utilize an existing off-the-shelf radioactive heat unit (called the 1-Watt RHU) which was developed by the Los Alamos National Laboratory, Los Alamos, N. Mex. The 1-Watt RHU produces 1 Watt of thermal power and is used in space to heat instruments so that they will not freeze. The radioactive element in the 1-Watt RHU is Pu-238, which is a very dangerous material, but many tests have proven that the containment vessel, which contains and protects the Pu-238, is so reliable that it would survive an explosion on lift-off and the maximum possible heating on reentry.
Studies have been conducted to combine existing thermoelectric modules with this heat source to produce about 40 mW of electric power at 5.5 Volts for space uses. No such module exists. In fact, one study concluded that the "prospects . . . [of satisfying these requirements] based on a 1-Watt heat source are not encouraging". (Schock and Or, Parametric Design Study of "Power Stick" and Its Derivatives, CONF 95011, American Institute of Physics, 1995)
What is needed is a miniature thermoelectric module capable of reliably producing for many years 10's of milliwatt power from a very low power heat source such as a heat source generating heat at a fraction of a Watt to a few Watts.